1. Field of the Invention
The present invention relates to a magnetic developer. More particularly, the present invention relates to a magnetic developer which is prominently excellent in the property being electrically charged by mutual friction of developer particles and which can provide a clear and sharp image having a high density.
2. Description of the Prior Art
As the developer capable of developing an electrostatic latent image without using a particular carrier, there is known a so-called one-component type magnetic developer comprising a powder of a magnetic material contained in developer particles.
As one type of this one-component magnetic developer, there is known a so-called conductive magnetic developer in which a fine powder of a magnetic material is incorporated in developer particles to impart a property of being magnetically attracted and a conducting agent such as carbon black is distributed on the surfaces of the particles to impart them electrically conductive (see, for example, the specifications of U.S. Pat. No. 3,689,245 and U.S. Pat. No. 3,965,022). When this conductive magnetic developer is brought in the form of a so-called magnetic brush into contact with an electrostatic latent imagecarrying substrate to effect development of the latent image, there can be obtained an excellent visible image free of a so-called edge effect or fog. However, as is well known, when the developer image is transferred to an ordinary transfer sheet from the substrate, a serious problem arises. More specifically, as described in Japanese Patent Application Laid-Open Specification No. 117435/75, when the inherent electric resistance of a transfer sheet used is lower than 3.times.10.sup.13 .OMEGA.-cm as in case of plain paper, broadening of contour or reduction of the transfer efficiency is caused by scattering of developer particles at the transfer step. This disadvantage is moderated to some extent by coating the toner-receiving surface of the transfer sheet with a resin, wax or oil having a high electric resistance. This improvement, however, is reduced under a high-humidity condition. Furthermore, the cost of the transfer sheet is increased by coating with a resin or the like and the feel of the transfer sheet is reduced.
As another type of the one-component magnetic developer, there is known a non-conductive magnetic developer comprising an intimate particulate mixture of a fine powder of a magnetic material and an electroscopic binder. For example, the specification of U.S. Pat. No. 3,645,770 discloses an electrostatic photographic reproduction process in which a magnetic brush (layer) of the above-mentioned non-conductive magnetic developer is charged with a polarity opposite to the polarity of the charge of an electrostatic latent image to be developed by means of corona discharge, the charged developer is brought into contact with a latent image-carrying substrate to develop the latent image and the developer image is transferred onto a transfer sheet. This electrostatic photographic reproduction process is advantageous in that a transfer image can be formed even on plain paper as the transfer sheet. However, this process is still disadvantageous in that it is difficult to uniformly charge the magnetic brush of the non-conductive magnetic developer even to the base portion thereof, it is generally difficult to form an image having a sufficient density and the apparatus become complicated because a corona discharge mechanism should be disposed in the developing zone.
Recently, there have been proposed a process in which an electrostatic latent image is developed by frictional charging of a non-conductive magnetic developer by frictional contact of the developer with the surface of a latent image-carrying substrate (see Japanese Patent Application Laid-Open Specification No. 62638/75) and a process in which development is effective by utilizing dielectric polarization of a non-conductive magnetic developer (see Japanese Patent Application Laid-Open Specification No. 133026/76). In the former process, however, if development conditions are not strictly controlled, fogging is readily caused (especially when the degree of the contact of the tip of the spike of magnetic toner particles with the surface of the photosensitive material is high) or fixing or blocking of the magnetic toner particles onto the developing sleeve is caused, and this undesirable phenomenon is especially conspicuous when the copying operation is conducted continuously. In the latter process, there does not arise the problem of fogging, but since a visible image is formed by developing a latent image by utilizing the dielectric polarizing effect induced in the magnetic toner, the low-potential area of the latent image is not effectively developed. Accordingly, in the resulting print, a low-density portion of an original is hardly reproduced and reproduction of a half tone is difficult. Moreover, prints obtained according to these two processes are poor in the image sharpness, and when a p-type photosensitive material such as selenium is used as the photosensitive plate and a positively charged image is developed, it is very difficult to obtain an image having a sufficient density according to any of the foregoing two processes.
Furthermore, the specification of U.S. Pat. No. 4,102,305 discloses a process in which a one-component type magnetic developer, the electric resistance of which changes depending on the intensity of the electric field, namely a one-component type magnetic developer which becomes substantially conductive in a high electric field but has a high electric resistance in a low electric field, is used, a high voltage is applied between a magnetic brush-forming sleeve and a photosensitive plate to effect development under such conditions that the developer particles become conductive and transfer of the developer particles to a transfer sheet is carried out in a low electric field or in an electric field-free state to obtain an excellent transferred image. This specification teaches that the above-mentioned developer having a high electric field dependency of the electric resistance is prepared by spray-granulating 50% by weight of stearate-coated magnetite and 50% by weight of a styrene/n-butyl methacrylate copolymer. This process is excellent in the above idea of obtaining a good transferred image, but this process is disadvantageous in that a peculiar high voltage apparatus is necessary for the development and though the formed image has a high density, the image sharpness is still insufficient.
Moreover, the specification of U.S. Pat. No. 4,121,931 discloses a process in which an electrically insulating one-component type magnetic developer is used, a magnetic brush-forming sleeve is used as an electrode and a voltage is applied between this electrode and a photosensitive plate to cause a turbulent agitation in the developer on the sleeve, whereby the developer particles are uniformly charged. This process, however, is disadvantageous in that a high voltage apparatus should be disposed in the developing zone and special means should be disposed to agitate the developer particles on the sleeve.
As will be apparent from the foregoing description, the conventional researches made on one-component type magnetic developers and developing processes using these developers are concentrated to the composition of the developer, the developer-preparing process and the process for charging developer particles, but properties of magnetite to be incorporated into the developer have hardly been studied.
Ordinarily, when a magnetic brush of a one-component type developer is brought into contact with the surface of an electrostatic latent image-carrying substrate, the individual developer particles receive an electrostatic attracting force (Coulomb force) acting between the developer particles and the electrostatic latent image and a magnetic attracting force acting between the developer particles and a magnetic brush-forming magnet. The developer particles on which the Coulomb force is larger are attracted to the electrostatic latent image, while the developer particles on which the magnetic attrating force is larger are attracted to the magnetic sleeve, with the result that development is effected according to the electrostatic latent image on the substrate. Therefore, it is required for the one-component type magnetic developer that a certain balance should be maintained between magnetic characteristics and charging characteristics at the development step. Accordingly, it will readily be understood that the characteristics of the magnetic material powder used for the one-component type magnetic developer have important influences on the characteristics of an image which will be formed.